Carbonless copy papers are those capable of producing an image upon the application of pressure as delivered by an impact device such as a typewriter or printer, or by a stylus such as a pencil or pen. Normally, such papers function by the transfer of a colorless reactant from a donor to a receptor, the receptor containing a coreactant capable of forming a color with the doner material. The conventional construction consists of solutions of dye precursors encapsulated in a shell coated on a donor sheet, with the dye developer being similarly coated on a receptor sheet.
Such papers may be coated with the color precursor on the back (CB), with the color precursor on the back and the color developer on the front of the same sheet (CFB), or with the color developer on the front of the sheet (CF). The separate sheets of the carbonless paper set are combined with the paper being arranged (from top to bottom) in terms of a CB, CFB, and CF, such that in each case a color former and a color developer will be brought into contact when the microcapsules containing the color-forming material are ruptured by pressure application. A variation on the use of CB, CFB, and CF papers is a self-contained (SC) carbonless paper wherein both color former and color developer materials are applied to the same side of a sheet or incorporated into the fiber lattice of the paper itself.
Carbonless papers are widely used in the forms industry. Typically, preprinted forms are compiled into a set or packet such that marking the top form will provide the required number of duplicates. In one instance, the carbonless paper is prepared in precollated sets wherein sheets of various colors and surfaces are packaged in reverse sequence sets wherein the sheets are arranged opposite to their normal functional order. That is, the CF sheet is first in the set with the CB sheet being last, with the required number of CFB sheets therebetween. When the sheets are then printed in a printer which automatically reverses their sequence in the delivery tray, they will end up in the proper functional order for subsequent data entry. Where reversal of the sequence in the delivery tray does not occur, precollated sheets can be arranged in their normal functional order.
Traditionally, carbonless paper forms have been printed by conventional printing techniques, such as offset lithography, etc. With the advent of high speed electrostatic copiers having dependable, high capacity collating systems, has come the natural attempts to print carbonless paper by such techniques. Such attempts have encountered problems because, for example, the base sheets upon which the coatings are to be applied to form carbonless papers conventionally imaged via offset printing do not have sufficient stiffness or sufficiently low sensitivity to machine conditions for curl and moisture control to be handled in the copier processors or sorters.
Yet another problem encountered when using high speed copiers, such as the Xerox 9000 series, is the development of specks of toner powder on the copies after a number of sheets have been printed. Such specking typically arises from photoreceptor spotting by toner particles which have been plasticized due to contact with solvents from ruptured microcapsules. Carbonless paper having microcapsules coated thereon are subject to premature rupture of the capsules when subjected to pressure, and high speed copiers typically apply pressure to the sheets in three separate stages within the machine operation.
The first location is at the feed assembly station. Abrasion and resultant capsule rupture occur due to friction feeding between feed and retard belts and then as the paper is nipped between steel and polymeric rollers. A common mode of contamination at this location is from the buildup of capsule detritus on the steel roll which later can flake off and transfer into the copying machine itself. Such flakes manifest themselves as large irregularly shaped spots on the printed forms, and usually appear after from approximately 20,000 to 40,000 copies have been run on the machine.
The second location is at the toner transfer site; the paper travels between a photoreceptor belt and a bias transfer roll where it is subject to sheer and pressure forces. It is thus important to have the copying machine in proper adjustment at this location to minimize such forces, which are obviously particularly detrimental to capsule integrity.
Rupture causes release of the encapsulated solution of the color precursor; the released solvent thus wets the surface of the bias transfer roll, thus coming in contact with toner. Such toners are typically made of a pigment such as carbon black in a polymer such as a styrene-butyl methacrylate copolymer. Such materials can be readily plasticized by the color precursor solvent to a soft tacky state. This causes the plasticized toner to adhere and transfer to the selenium photoreceptor, thence the paper in background areas to form specks of about 200 to 300 microns in diameter.
The third location where pressure is applied to the paper during the printing process is at the heat/pressure fixation station. Here the surface temperature of the paper reaches about 160.degree. F. and the pressure is believed to be about 1200 psi, which pressure can again cause capsule rupture, with resultant reduced performance of the carbonless system.
Yet another problem encountered with carbonless paper in imaging via high speed copiers is lint and paper dust which may act as a nucleus for toner transfer in background areas.
Treier, in U.S. Pat. No. 4,046,404, assigned to Xerox Corp., teaches the use of hollow spheres dispersed within the paper to increase the stiffness and caliper thereof when a carbonless paper substrate is sought. His effort is directed to making light weight paper, rejecting the use of heavier basis weight stock. Furthermore, his coating formulation includes starch granules as a cushioning agent in the capsule-containing coating, designed to protect or cushion the imaging capsules to prevent their premature breaking.
Stolfo, in U.S. Pat. No. 4,398,954, was particularly concerned with the oil contamination of the transfer roll in a copier; he incorporated finely divided oleophilic silica with the capsule coating on the donor sheet. The silica apparently adsorbed the solvents released when capsules ruptured inadvertently. He also teaches the inclusion of starch particles to act as a cushioning agent to minimize capsule rupture (often termed "stilt" material). He also recognized the importance of capsule size in reducing the risk of capsule breakage and the deleterious release of oil to afford contamination within the copier. Thus, he discloses the use of a mean capsule size of approximately 4 microns.
A satisfactory solution to the foregoing problems still has not been found, however, as evidenced by Xerox Corp. publications.
I have now discovered that the foregoing problems can surprisingly be resolved through the use of high basis weight bond paper coated with small capsules, without the necessity of any cushioning or stilt material, and without the necessity for addition of a material to adsorb oil released when inadvertent rupture of capsules occurs. Despite the use of small capsules and high basis weight bond paper, the images formed upon application of pressure are dense and readily legible.